This application examines the nature of syncytia induced by the gp120-CD4 interaction in HIV infected cells. Current dogma would say that syncytia are only giant disorganized cell bodies with short life spans and that they do not play an active role in HIV pathogenesis. However, by combining immunofluorescent, electron microscopic and computer assisted analysis of syncytia, the P.I. has discovered that syncytia formed in vitro by infected Sup T1 cells or peripheral blood T-cells 1) reorganize their cytoskeleton and rearrange their nuclei to mimic the organization of a single cell; 2) extend a single giant pseudopod in a fashion similar to a single cell; 3) crawl along a variety of substrates with the same velocity as single cells; 4) invade and disorganize collagen and endothelial monolayers through the extension of giant pseudopods; 5) phagocytose as well as fuse with T cells by coiling pseudopods around T cells; 6) release a potent T cell chemoattractant; and 7) can live for as long as 5.5 days. Preliminary studies with lymph node material from an HIV-infected individual have identified 9 syncytia which seem to exhibit at least some of the properties described in vitro. In this application, the P.I. proposes to continue his detailed studies with syncytia derived in vitro, as well as spend a considerable amount of effort studying syncytia obtained from lymph node biopsies of HIV-infected individuals. He believes that this "two tiered" approach is necessary since the availability of patient material limits what can be done with lymph-node derived syncytia. The in vitro studies will include 1) Detailed observations of the behavior of cells during fusion; 2) Testing the dependency of each step of fusion on actin polymerization; 3) Characterizing syncytium mediated chemotaxis; 4)Testing known chemoattractants with T cells; 5) Characterizing the chemoattractant released from syncytia and generating antiserum to it; 6) Testing if antibody to the chemoattractant or high concentrations of chemoattractant can block syncytia formation; 7) Studying how syncytia phagocytose T-cell; 8) Studying how syncytia disrupt collagen and endothethial monolayers; 9) Extending the in vitro work to macrophages. Once the perimeters are established with cells in vitro , syncytia derived from lymph nodes of infected patients will be studied in a similar fashion.